1. Field of the Invention
The invention relates to a high pressure mercury lamp. The invention relates especially to a super high pressure mercury lamp in which a discharge vessel is filled with at least 0.16 mg/mm.sup.3 of mercury, in which the mercury vapor pressure during operation is at least equal to 110 atm, and which is used to back light a liquid crystal display device or the like.
2. Description of the Related Art
In a liquid crystal display device of the projection type, there is a need for illumination of images on a rectangular screen in a uniform manner and with adequate color reproduction. Therefore, as the light source, a metal halide lamp is used which is filled with mercury and metal halides. The metal halide lamps have recently been made even smaller so that more and more they represent point light sources. Metal halide lamps with an extremely small distance between the electrodes are used in practice.
Proceeding from this background, instead of metal halide lamps, recently lamps have been suggested with an extremely high mercury vapor pressure which is, for example, at least equal to 200 bar (roughly 197 atm). Here, by increasing the mercury vapor pressure, spreading of the arc is suppressed (concentrated), and furthermore, there is an effort to increase light intensity even more. These lamps are disclosed, for example, in Japanese patent disclosure document HEI 2-148561 and corresponding U.S. Pat. No. 5,109,181, and Japanese patent disclosure document HEI 6-52830 and corresponding U.S. Pat. No. 5,497,049.
In U.S. Pat. No. 5,109,181, a high pressure mercury lamp is disclosed in which a discharge vessel provided with a pair of tungsten electrodes is filled with a rare gas, at least 0.2 mg/mm.sup.3 of mercury, and a halogen in the range from 1.times.10.sup.-6 to 1.times.10.sup.-4 .mu.mole/mm.sup.3. This lamp is operated with a wall load that is at least equal to 1 W/mm.sup.2. The reason for adding an amount of mercury at least equal to 0.2 mg/mm.sup.3 is to improve color reproduction by increasing the mercury pressure and the continuous spectrum in the area of visible radiation, especially in the red range. The reason for a wall load that is at least equal to 1 W/mm.sup.2 is the need for a temperature increase in the coolest portion in order to increase the mercury pressure. The reason for adding the halogen is to prevent blackening of the envelope; this can be obtained from the patent. However, the reason for fixing the amount of the halogen in the range from 1.times.10.sup.-6 to 1.times.10.sup.-4 .mu.mole/mm.sup.3 is not described. Furthermore, it is also described that the halogen is added in the form of methylene bromide (CH.sub.2 Br.sub.2).
On the other hand, U.S. Pat. No. 5,497,049, it is described that, in addition to the above described amount of mercury, values of wall load, amount of halogen, the shape of the discharge vessel and the distance between the electrodes are fixed, and furthermore, bromine is used as the halogen. The reason for adding bromine is to prevent blackening of the envelope. When at least 10.sup.-6 .mu.mole/mm.sup.3 of bromine is added, a sufficient effect is obtained. Furthermore, it is shown that the electrodes are etched when more than 10.sup.-4 .mu.mole/mm.sup.3 of bromine is added. Furthermore, it is described that this lamp is suitable for a projector light source and that the degree to which illuminance of the screen of a liquid crystal projection television is maintained is better than in a conventional lamp.
However, based on the specifications disclosed in the above described prior art, a host of lamps was produced, installed in a liquid crystal projector and experiments were run with respect to the illuminance of the screen. As a result, it became apparent that, in reality, after operating the lamps for a few hundred hours, the illuminance of the screen was greatly reduced.
This reduction in the radiant light intensity was a result of milky opacification of part of the discharge vessel. Furthermore, the milky opacification increases quickly, once it has occurred in part of the discharge vessel. Formation and spreading of this milky opacification lead to blackening of the envelope, and furthermore, deformation and wear of the tip of the electrodes also occur. It was found that, by synergistic effects, a reduction of illuminance of the screen is caused.
In this case, the mechanism of formation of milky opacification in the discharge vessel and the spreading of resulting milky opacification is not entirely clear. As a result of the studies collected and checked by the inventors, however, the following is assumed.
In a discharge in a mixed gas of mercury vapor with an extremely high pressure, the amount of the mercury added being at least equal to 0.16 mg/mm.sup.3, and the rare gas yields excimer light from mercury rare gas in a wavelength range between the rare gas excimer light and a mercury resonance line with a wavelength of 185 nm. If Ar, Kr, and Xe are used as the rare gas, rare gas excimer light is formed at wavelengths of roughly 126 nm, 146 nm and 172 nm, respectively. Since the mercury pressure is very high, the line width of the resonance line of the mercury atoms with a 185 nm wavelength becomes larger. The light intensity of the wavelengths which are shorter than the resonance line is intensified to a relative degree. At the same time, mercury rare gas excimer light is formed between the rare gas excimer light and the 185 nm wavelength light.
In this super high pressure mercury lamp, the excimer light is emitted extremely effectively by the rare gas (light with wavelengths of 126 nm, 146 nm, and 172 nm), as is the light with the wavelengths which are shorter than the resonance line of the mercury atoms with a 185 nm wavelength, and the mercury rare gas excimer light (hereinafter, this light is called "UV radiation with short wavelengths") in the band area of roughly 126 nm to 185 nm. This UV radiation with short wavelengths on the inside of the discharge vessel has extremely high irradiance because the wall load of the discharge vessel is high.
On the other hand, there is a tendency for the wavelength range in which absorption takes place by the fused silica glass which forms the discharge vessel to be shifted in the direction toward longer wavelengths when the temperature of the discharge vessel becomes high. In a high pressure mercury lamp with a high value of the wall load that is at least equal to 0.8 W/mm.sup.2, the fused silica glass has a very high temperature by which the emitted UV radiation with short wavelengths is absorbed by the fused silica glass.
This means that, in a mercury lamp with an extremely high mercury vapor pressure and extremely high wall load, UV radiation with short wavelengths is emitted in an intensity which is not comparable to UV radiation with short wavelengths in a conventional mercury lamp, and this UV radiation with short wavelengths is in a state in which it is easily absorbed by the fused silica glass.
If the above described UV radiation with short wavelengths is absorbed by the fused silica glass, the bond of silicon (Si) to oxygen (O) which comprises the fused silica glass is destroyed, resulting in strain stress, and thus, a fundamental change of the surface composition of the fused silica glass surface. Irradiation with UV radiation with short wavelengths causes vaporization of the Si or SiO comprising the fused silica glass, and the Si or SiO is adsorbed on the immediately adjacent fused silica glass surface. In the case of a large amount of absorbed UV radiation with short wavelengths, therefore, on the fused silica glass surface fine convex or concave points form, presumably causing the milky opacification.
In this case, the amount of absorption of UV radiation with short wavelengths is relatively small in the state in which the fused silica glass surface is clean. However, there is a tendency for the amount of absorption to become greater, the more impurities are present. Therefore, it is desirable, during lamp operation; for control to be effected such that the inner surface of the fused silica glass has no impurities. However, for this reason, it is necessary to avoid, as much as possible, mixing substances which cause impurities in the discharge vessel during the lamp production process.
Here, carbon is a contaminating substance which can be especially difficult to handle because, in the lamp production environment, it exists in the form of different organic compounds.
When, in one part of the fused silica glass, milky opacification forms, the heat is absorbed by multipath reflection of the light which contains infrared radiation, resulting in the temperature of the milky opacified parts rising. As a result, the light absorbed by the fused silica glass shifts in the direction toward longer wavelengths, leading to even more acceleration of absorption of the UV radiation with short wavelengths by the fused silica glass. It can be imagined that, as a result, the formation of the fine convex or concave points is accelerated, and therefore, that the milky opacification quickly spreads.
Furthermore, Si or SiO vaporizes from the tube wall when the Si and O bond of the fused silica glass is destroyed by UV irradiation. The vaporized Si or SiO is adsorbed by electrode tips and reduces the melting point of tungsten; this causes deformation and wear of the electrode tips and blackening of the envelope by tungsten.